Adds a new 'key.retrieve_symbol_graph' option to the request. When set to 1 it
includes the JSON for a SymbolGraph containing a single node for the symbol at
the requested position.
This also extends the SymbolGraph library with a new entry point to get a graph
for a single symbol, and to additionally support type substitution to match the
existing CursorInfo behavior (e.g. so that when invoked on `first` in
`Array<Int>().first`, the type is given as `Int?` rather than `Element?`).
Resolves rdar://problem/70551509
